Double metal cyanide complex compounds are well known catalysts for epoxide polymerization. The catalysts are highly active, and give polyether polyols that have low unsaturation compared with similar polyols made using basic (KOH) catalysis. Conventional DMC catalysts are prepared by reacting aqueous solutions of metal salts and metal cyanide salts to form a precipitate of the DMC compound. The catalysts can be used to make a variety of polymer products, including polyether, polyester, and polyetherester polyols. Many of the polyols are useful in various polyurethane coatings, elastomers, sealants, foams, and adhesives.
Conventional DMC catalysts are usually prepared in the presence of a low molecular weight complexing agent, typically an ether such as glyme (dimethoxyethane) or diglyme. The ether complexes with the DMC compound, and favorably impacts the activity of the catalyst for epoxide polymerization. In one conventional preparation, aqueous solutions of zinc chloride (excess) and potassium hexacyanocobaltate are combined by simple mixing. The resulting precipitate of zinc hexacyanocobaltate is then mixed with aqueous glyme. An active catalyst is obtained that has the formula: EQU Zn.sub.3 [Co(CN).sub.6 ].sub.2 xZnCl.sub.2 yH.sub.2 OzGlyme
Other known complexing agents include alcohols, ketones, esters, amides, ureas, and the like. (See, for example, U.S. Pat. Nos. 3,427,256, 3,427,334, 3,278,459, and Japanese Pat. Appl. Kokai Nos. 4-145123, 3-281529 and 3-149222). Generally, the catalyst made with glyme has been the catalyst of choice. The catalysts have relatively high surface areas, typically within the range of about 50-200 m.sup.2 /g.
Normally, the complexing agent is added to the reaction mixture following precipitation of the DMC compound. Some references, such as U.S. Pat. No. 5,158,922, indicate that the complexing agent can be included with either or both of the aqueous reactant solutions, but no reference teaches any particular advantage of having the complexing agent present in the reactant solutions.
Double metal cyanide compounds prepared in the absence of a complexing agent are highly crystalline by X-ray diffraction analysis (See FIG. 4), and are inactive for epoxide polymerization. When the complexing agents described above are used, the resulting catalysts actively polymerize epoxides. Our X-ray diffraction analyses of active DMC complexes prepared according to methods known in the art suggest that conventional DMC catalysts are actually mixtures of a highly crystalline DMC compound and a more amorphous component. Typically, conventional DMC catalysts--which are generally prepared by simple mixing--contain at least about 35 wt. % of highly crystalline DMC compound. DMC compounds useful as epoxide polymerization catalysts and containing less than about 30 wt. % of highly crystalline DMC compound are not known.
Double metal cyanide catalysts generally have good activity for epoxide polymerizations, often much greater than conventional basic catalysts. However, because the DMC catalysts are rather expensive, catalysts with improved activity are desirable because reduced catalyst levels could be used.
Double metal cyanide catalysts normally require an "induction" period. In contrast to basic catalysts, DMC catalysts ordinarily will not begin polymerizing epoxides immediately following exposure of epoxide and starter polyol to the catalyst. Instead, the catalyst needs to be activated with a small proportion of epoxide before it becomes safe to begin continuously adding the remaining epoxide. Induction periods of an hour or more are typical yet costly in terms of increased cycle times in a polyol production facility. Reduction or elimination of the induction period is desirable.
An advantage of DMC catalysts is that they permit the synthesis of high molecular weight polyether polyols having relatively low unsaturation. The adverse impact of polyol unsaturation on polyurethane properties is well documented. (See, for example, C. P. Smith et al., J. Elast. Plast., 24 (1992) 306, and R. L. Mascioli, SPI Proceedings, 32nd Annual Polyurethane Tech./Market. Conf. (1989) 139.) When a DMC catalyst is used, polyols having unsaturations as low as about 0.015 meq/g can be made. Polyether polyols with even lower unsaturations can be made if a solvent such as tetrahydrofuran is used to make the polyol. See, for example, U.S. Pat. Nos. 3,829,505 and 4,843,054. However, for commercial polyol production, the use of a solvent is not particularly desirable. Thus, other ways to further reduce polyol unsaturation are needed.
When conventional DMC catalysts are used to polymerize epoxides, the polyether polyol products contain relatively low levels (about 5-10 wt. %) of low molecular weight polyol impurities. A way to eliminate these polyol impurities is desirable because improved polyurethanes could result from the use of more monodisperse polyols.
Double metal cyanide complex catalyst residues are often difficult to remove from polyether polyols, and a wide variety of methods have been developed to cope with the problem. Removal of DMC catalyst residues from the polyols promotes long-term storage stability and consistent polyol performance in urethane formulation. Most methods involve some kind of chemical treatment of the polyol following polymerization. There has been little progress made in developing catalyst preparation methods that ultimately facilitate catalyst removal from the polyol products.